System for determining the status of an entity in a computer network

ABSTRACT

A network management system includes a user interface, a virtual network and a device communication manager. The virtual network includes models which represent network entities and model relations which represent relations between network entities. Each model includes network data relating to a corresponding network entity and one or more inference handlers for processing the network data to provide user information. The system can poll or communicate with certain network entities and can infer the status of network connectors and other network entities for which polling is impossible or impractical. The system performs a fault isolation technique wherein the fault status of a network device is suppressed when it is determined that the device is not defective. User displays include hierarchical location views and topological views of the network configuration. Network devices are represented on the displays by multifunction icons which permit the user to select additional displays showing detailed information regarding different aspects of the corresponding network device.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 08/355,430,filed Dec. 13, 1994, U.S. Pat. No. 5,559,955 which is a continuation ofapplication Ser. No. 08/216,696 filed Mar. 23, 1994, now abandoned,which is a continuation of application Ser. No. 07/797,121, filed Nov.22, 1991, now abandoned which is a continuation-in-part of applicationSer. No. 07/588,509, filed Sep. 17, 1990, now abandoned.

FIELD OF THE INVENTION

This invention relates to systems for management of computer networksand, more particularly, to methods and apparatus for monitoring thestatus of a network entity without communicating with the networkentity.

BACKGROUND OF THE INVENTION

Computer-networks are widely used to provide increased computing power,sharing of resources and communication between users. Computer systemsand computer system components are interconnected to form a network.Networks may include a number of computer devices within a room,building or site that are interconnected by a high speed local data linksuch as local area network (LAN), token ring, Ethernet, or the like.Local networks in different locations may be interconnected bytechniques such as packet switching, microwave links and satellite linksto form a world-wide network. A network may include several hundred ormore interconnected devices.

In computer networks, a number of issues arise, including trafficoverload on parts of the network, optimum placement of networkresources, security, isolation of network faults, and the like. Theseissues become more complex and difficult as networks become larger andmore complex. For example, if a network device is not sending messages,it may be difficult to determine whether the fault is in the networkdevice itself, the data communication link or an intermediate networkdevice between the sending and receiving network devices.

Network management systems have been utilized in the past in attempts toaddress such issues. Prior art network management systems typicallyoperated by remote access to and monitoring of information from networkdevices. The network management system collected large volumes ofinformation which required evaluation by a network administrator. Priorart network management systems place a tremendous burden on the networkadministrator. He must be a networking expert in order to understand theimplications of a change in a network device parameter. Theadministrator must also understand the topology of each section of thenetwork in order to understand what may have caused the change. Inaddition, the administrator must sift through reams of information andfalse alarms in order to determine the cause of a problem.

It is therefore desirable to provide a network management system whichcan systematize the knowledge of the networking expert such that commonproblems can be detected, isolated and repaired, either automatically orwith the involvement of less skilled personnel. Such a system must havecertain characteristics in order to achieve this goal. The system musthave a complete and precise representation of the network and thenetworking technologies involved. It is insufficient to extend prior artnetwork management systems to include connections between devices. Anetwork is much more than the devices and the wires which connect them.The network involves the network devices, the network protocols and thesoftware running on the devices. Without consideration of these aspectsof the network, a model is incomplete. A system must be flexible andextendable. It must allow not only for the modeling of new devices, butmust allow for the modeling of new technologies, media applications andprotocol. The system must provide a facility for efficientlyencapsulating the expert's knowledge into the system.

It is a general object of the present invention to provide improvedmethods and apparatus for managing networks.

It is another object of the present invention to provide networkmanagement systems which utilize models of network entities andinterrelationships between network entities.

It is a further object of the present invention to provide methods andapparatus for inferring information about network entities with whichcommunication is impossible or impractical.

It is a further object of the present invention to provide methods andapparatus for determining the operational status of network devices,such as connectors, cables, buses, and the like, that are not capable ofcommunication.

SUMMARY OF THE INVENTION

According to the present invention, these and other objects andadvantages are achieved in a method for managing a network comprisinginterrelated network entities including communicating network entitiesand at least one non-communicating network entity. The method of thepresent invention permits the status of network entities such as coaxialsegments, connectors, buses, rings, optical fiber segments, wide areasegments, multiplexers, and the like, to be monitored by the networkmanagement system, even though such network entities cannot be directlypolled. In accordance with the invention, the method for monitoring thestatus of the non-communicating network entity comprises the steps ofproviding an electronic network management system, the networkmanagement system communicating with the communicating network entitiesto obtain operational information as to the communicating networkentities, and the network management system using the operationalinformation as to the communicating network entities and information asto relations between the non-communicating network entity and thecommunicating network entities to infer the status of thenon-communicating network entity. The status of the non-communicatingnetwork entity is inferred from information as to network entitiesconnected to the non-communicating network entity.

The network management system preferably includes a representation ofthe network, including models and relations between models. The modelsinclude models of communicating network entities and inferred models ofnon-communicating network entities. Each model includes inferencehandlers. The inference handlers in the inferred models update statusinformation in response to information obtained from the models of thecommunicating network entities.

When all network entities connected to the non-communicating networkentity have a lost contact status, the contact status of thenon-communicating network entity is inferred as lost. When at least onenetwork entity connected to the non-communicating network entity has anestablished contact status, the contact status of the non-communicatingnetwork entity is inferred as established. The contact status of thenon-communicating network entity can be provided to a user in the samemanner as information relating to communicating network entities.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the accompanying drawings which are incorporated herein byreference and in which:

FIG. 1 is a block diagram of a network management system in accordancewith the invention;

FIG. 2 is a block diagram showing an example of a network;

FIG. 3 is a schematic diagram showing the structure of models and therelations between models;

FIG. 4 is a block diagram showing a portion of the representation of thenetwork of FIG. 2 in the virtual network machine;

FIG. 5 is a flow chart illustrating an example of operation of thevirtual network machine;

FIG. 6 is a flow chart of a fault isolation technique in accordance withthe present invention;

FIGS. 7A, 7B, and 7C show examples of location display views provided bythe network management system;

FIGS. 8A and 8B show examples of topological display views provided bythe network management system;

FIG. 9 is a schematic diagram of a multifunction icon employed in theuser display views; and

FIG. 10 shows an example of an alarm log display provided by the networkmanagement system.

FIGS. 11, 12 and 13 are block diagrams of a network which illustrate theoperation of an inferred connector model of a coaxial segment.

DETAILED DESCRIPTION OF THE INVENTION

A block diagram of a network management system in accordance with thepresent invention is shown in FIG. 1. The major components of thenetwork management system are a user interface 10, a virtual networkmachine 12, and a device communication manager 14.

The user interface 10, which may include a video display screen,keyboard, mouse and printer, provides all interaction with the user. Theuser interface controls the screen, keyboard, mouse and printer andprovides the user with different views of the network that is beingmanaged. The user interface receives network information from thevirtual network machine 12.

The virtual network machine 12 contains a software representation of thenetwork being managed, including models that represent the devices andother entities associated with the network, and relations between themodels. The virtual network machine 12 is associated with a databasemanager 16 which manages the storage and retrieval of disk-based data.Such data includes configuration data, an event log, statistics, historyand current state information.

The device communication manager 14 is connected to a network 18 andhandles communication between the virtual network machine 12 and networkdevices. The data received from the network devices is provided by thedevice communication manager to the virtual network machine 12. Thedevice communication manager 14 converts generic requests from thevirtual network machine 12 to the required network management protocolfor communicating with each network device. Existing network managementprotocols include Simple Network Management Protocol (SNMP), InternetControl Message Protocol (ICMP) and many proprietary network managementprotocols. Certain types of network devices are designed to communicatewith a network management system using one of these protocols.

A view personality module 20 connected to the user interface 10 containsa collection of data modules which permit the user interface to providedifferent views of the network. A device personality module 22 connectedto the virtual network machine 12 contains a collection of data moduleswhich permit devices and other network entities to be configured andmanaged with the network management system. A protocol personalitymodule 24 connected to the device communication manager contains acollection of data modules which permit communication with all devicesthat communicate using the network management protocols specified by themodule 24.

The personality modules 20, 22 and 24 provide a system that is highlyflexible and user configurable. By altering the personality module 20,the user can specify customized views or displays. By changing thedevice personality module 22, the user can add new types of networkdevices to the system. Similarly, by changing the protocol personalitymodule 24, the network management system can operate with new ordifferent network management protocols. The personality modules permitthe system to be reconfigured and customized without changing the basiccontrol code of the system.

The overall software architecture of the present invention is shown inFIG. 1. The hardware for supporting the system of FIG. 1 is typically aworkstation such as a Sun Model 3 or 4, or a 386 PC compatible computerrunning Unix. In this embodiment, a minimum of 8 megabytes of memory isrequired with a display device which supports a minimum of 640×680pixels×256 color resolution. The basic software includes a Unix releasethat supports sockets, X-windows and Open Software Foundation Motif 1.0.The network management system of the present invention is implementedusing the C++ programming-language, but could be implemented in otherobject-oriented languages such as Eiffel, Smalltalk, ADA, or the like.The virtual network machine 12 and the device communication manager 14may be run on a separate computer from the user interface 10 forincreased operating speed or increased reliability. It is understoodthat the present invention is in no way limited to any particularhardware, e.g., Sun Model 3 or 4, or operating system, e.g., Unix.

An example of a network is shown in FIG. 2. The network includesworkstations 30, 31, 32, 33 and disk units 34 and 35 interconnected by adata bus 36. Workstations 30 and 31 and disk unit 34 are located in aroom 38, and workstations 32 and 33 and disk unit 35 are located in aroom 40. The rooms 38 and 40 are located within a building 42. Networkdevices 44, 45 and 46 are interconnected by a data bus 47 and arelocated in a building 48 at the same site as building 42. The networkportions in buildings 42 and 48 are interconnected by a bridge 50. Abuilding 52 remotely located (in a different city, state or country)from buildings 42 and 48, contains network devices 53, 54, 55 and 56interconnected by a data bus 57. The network devices in building 52 areinterconnected to the network in building 48 by interface devices 59 and60, which may communicate by a packet switching system, a microwave linkor a satellite link. The network management system shown in FIG. 1 anddescribed above is connected to the network of FIG. 2 at any convenientpoint, such as data bus 36.

In general, the network management system shown in FIG. 1 performs twomajor operations during normal operation. It services user requestsentered by the user at user interface 10 and provides networkinformation such as alarms and events to user interface 10. In addition,the virtual network machine 12 polls the network to obtain informationfor updating the network models as described hereinafter. In some cases,the network devices send status information to the network managementsystem automatically without polling. In other cases the device modelsinfer status information from data gathered for other models. In anycase, the information received from the network is processed so that theoperational status, faults and other information pertaining to thenetwork are presented to the user in a systematized and organizedmanner.

As indicated above, the network entities that make up the network thatis being managed by the network management system are represented bysoftware models in the virtual network machine 12. The models representnetwork devices such as printed circuit boards, printed circuit boardracks, bridges, routers, hubs, models also represent The models alsorepresent locations or topologies. Location models represent the partsof a network geographically associated with a building, country, floor,panel, rack, region, room, section, sector, site or the world.Topological models represent the network devices that are topologicallyassociated with a local area network or subnetwork. Models can alsorepresent components of network devices such as individual printedcircuit boards, ports and the like. In addition, models can representsoftware applications such as data relay, network monitor, terminalserver and end point operations. In general, models can represent anynetwork entity that is of interest in connection with managing ormonitoring the network.

The virtual network machine includes a collection of models whichrepresent the various network entities. In a preferred embodiment, themodels themselves are collections of C++ objects. The virtual networkmachine also includes model relations which define theinterrelationships between the various models. Several types ofrelations can be specified. A "connects to" relation is used to specifyan interconnection between network devices. For example, theinterconnection between two workstations is specified by a "connects to"relation. A "contains" relation is used to specify a network entity thatis contained within another network entity. Thus for example, aworkstation model may be contained in a room, building or local networkmodel. An "executes" relation is used to specify the relation between asoftware application and the network device on which it runs. An "ispart of" relation specifies the relation between a network device andits components. For example, a port model may be part of a board modelor a card rack model.

Relations are specified as pairs of models, known as associations. Therelations can specify peer-to-peer associations and hierarchicalassociations.

Each model includes a number of attributes and one or more inferencehandlers. The attributes are data which define the characteristics andstatus of the network entity being modeled. Basic attributes include amodel name, a model type name, a model type handle, a polling interval,a next-time-to-poll, a retry count, a contact status, an activationstatus, a time-of-last-poll and statistics pertaining to the networkentity which is being modeled. Polling of network devices will bedescribed hereinafter. In addition, attributes that are unique to aparticular type of network device can be defined. For example, a networkbridge contains a table that defines the devices that are located oneach side of the bridge. A model of the network bridge can contain, asone of its attributes, a copy of the table.

In a preferred embodiment of the invention, each attribute contained ina model type includes the following:

(1) An attribute name that identifies the attribute.

(2) An attribute type that defines the kind of attribute. Attributetypes may include Boolean values, integers, counters, dates, textstrings, and the like.

(3) Attribute flags indicate how the attribute is to be manipulated. Amemory flag indicates that the attribute is stored in memory. A databaseflag indicates that the attribute is maintained in the database of thevirtual network machine. An external flag indicates that the attributeis maintained in the device being modeled. A polled flag indicates thatthe attributes value should be periodically surveyed or polled by thedevice being modeled. The flags also indicate whether the attribute isreadable or writable by the user.

(4) Object identifier is the identifier used to access the attribute inthe device. It is defined by the network management protocol used toaccess the device.

(5) Attribute help string is a text string which contains a descriptionof the defined attribute. When the user asks for help regarding thisattribute, the text string appears on the user interface screen.

(6) Attribute value is the value of the attribute.

The models used in the virtual network machine also include one or moreinference handlers. An inference handler in this embodiment is a C++object which performs a specified computation, decision, action orinference. The inference handlers collectively constitute theintelligence of the model. An individual inference handler is defined bythe type of processing performed, the source or sources of the stimulusand the destination of the result. The result is an output of aninference handler and may include attribute changes, creation ordestruction of models, alarms or any other valid output. The operationof the inference handler is initiated by a trigger, which is an eventoccurring in the virtual network machine. Triggers include attributechanges in the same model, attribute changes in another model, relationchanges, events, model creation or destruction, and the like. Thus, eachmodel includes inference handlers which perform specified functions uponthe occurrence of predetermined events which trigger the inferencehandlers.

A schematic diagram of a simple model configuration is shown in FIG. 3to illustrate the concepts of the present invention. A device model 80includes attributes 1 to x and inference handlers 1 to y. A device model82 includes attributes 1 to u and inference handlers 1 to v. A connectrelation 84 indicates that models 80 and 82 are connected in thephysical network. A room model 86 includes attributes 1 to m andinference handlers 1 to n. A relation 88 indicates that model 80 iscontained within room model 86, and a relation 90 indicates that model82 is contained within room model 86. Each of the models and the modelrelations shown in FIG. 3 is implemented as a C++ object. It will beunderstood that a representation of an actual network would be much morecomplex than the configuration shown in FIG. 3 and, as noted above, thatthe present invention is not limited to a particular programminglanguage.

As discussed above, the collection of models and model relations in thevirtual network machine form a representation of the physical networkbeing managed. The models represent not only the configuration of thenetwork, but also represent its status on a dynamic basis. The status ofthe network and other information and data relating to the network isobtained by the models in a number of different ways. A primarytechnique for obtaining information from the network involves polling.At specified intervals, a model in the virtual network machine 12requests the device communication manager 14 to poll the network devicewhich corresponds to the model. The device communication manager 14converts the request to the necessary protocol for communicating withthe network device. The network device returns the requested informationto the device communication manager 14, which extracts the deviceinformation and forwards it to the virtual network machine 12 forupdating one or more attributes in the model of the network device. Thepolling interval is specified individually for each model andcorresponding network device, depending on the importance of theattribute, the frequency with which it is likely to change, and thelike. The polling interval, in general, is a compromise between a desirethat the models accurately reflect the present status of the networkdevice and a desire to minimize network management traffic which couldadversely impact normal network operation.

According to another technique for updating the information contained inthe models, the network devices automatically transmit information tothe network management system upon the occurrence of significant eventswithout polling. This requires that the network devices bepre-programmed for such operation.

It will be understood that communication between a model and itscorresponding network entity is possible only for certain types ofdevices such as bridges, card racks, hubs, etc. In other cases, thenetwork entity being modeled is not capable of communicating its statusto the network management system. For example, models of buildings orrooms containing network devices and models of cables cannot communicatewith the corresponding network entities. In this case, the status of thenetwork entity is inferred by the model from information contained inmodels of other network devices. Since successful polling of a networkdevice connected to a cable may indicate that the cable is functioningproperly, the status of the cable can be inferred from informationcontained in a model of the attached network device. Similarly, theoperational status of a room can be inferred from the operational statuscontained in models of the network devices located within the room. Inorder for a model to make such inferences, it is necessary for the modelto obtain information from related models. In a function called a modelwatch, an attribute in one model is monitored or watched by one or moreother models. A change in the watched attribute may trigger inferencehandlers in the watching models.

The virtual network machine also includes an event log, a statistics logand an alarm log. These logs permit information contained in the modelsto be organized and presented to the user and to be recorded in thedatabase.

The event message provides specific information about events, includingalarms that have occurred in a given model. The events pass from themodel to an event log manager which records the event in the externaldatabase. An event message is also sent to the user interface based onevent filters, as discussed below. The user can request eventinformation from the database. An event message includes a model handle,a model-type handle, an event date and time, an event type and subtype,an event severity, a model name, a model-type name, an event user name,an event data count and event variable data. The event variable datapermits additional information to be provided about the event.

Event messages sent to the user interface can utilize a filter processthat is specified by the user. The user can specify model types and aminimum event severity for which events will be displayed on the userscreen. Events from unspecified model types or less than the minimumseverity will not be displayed. Many other event selection or filteringcriteria can be used. In general, any information contained in the eventmessage can be used for event filtering.

Statistics history messages are similar to the event messages describedabove. The statistics information includes any model parameters orfunctions which the user wishes to monitor. A statistics history messagepasses from the model to a statistics log manager and subsequently tothe external database. The statistics message is also sent to the userinterface based upon predefined filter parameters. The user can requestthe statistics log manager to obtain and display statistics informationfrom the external database. Statistics messages are compiled whenever adevice read procedure occurs.

When an alarm event occurs in a model, a notice of the alarm event issent to an alarm log and to the event log. The alarm log selects themost severe alarm for each model which is registering an alarm. Thealarms are sent to an alarm window in the user interface. The user canobtain more information on the alarm message by pressing an appropriatebutton on the window display. Alarm log messages include the followingparameters: alarm condition, alarm cause, alarm status, alarm securitydata, alarm clear switch and alarm unique ID.

An example will now be given to illustrate the operation of the virtualnetwork machine 12. A portion of the virtual machine 12 is shownschematically in FIG. 4. The models shown in FIG. 4 correspond tonetwork entities shown in FIG. 2. A flow chart illustrating the exampleis shown in FIG. 5. Each network device has a model in the virtualnetwork machine 12. Thus, for example, model 144 corresponds to networkdevice 44, model 145 corresponds to network device 45, etc. Models 144and 145 are related by connection relation 147 which corresponds to databus 47. Room model 148 is related to models 144 and 145 by a containsrelation.

In operation, at a specified time model 144 initiates polling of networkdevice 44 in step 200 in order to obtain an update of the status ofnetwork device 44. The model 144 sends a request to the devicecommunication manager 14 to poll network device 44. The devicecommunication manager 14 converts the request to the required protocolfor communication with network device 44 and sends the message. Therequested information may, for example, be the number of packets sent onthe network in a given time and the number of errors that occurred. Whenthe requested information is returned to model 144, the correspondingattributes in model 144 are updated in step 206 and an error rateinference handler is triggered. The error rate inference handler in step208 calculates the error rate for network device 44. If the error rateis within prescribed limits (step 210), an error rate attribute isupdated, and the new information is logged into the database (step 212).If the calculated error rate is above a predetermined limit, an erroralarm inference handler is triggered. The error alarm inference handlermay shut off the corresponding network device 44 and send an alarm tothe user interface in step 214. The alarm is also logged in thedatabase. If the network device 44 is shut off in response to a higherror rate, a condition attribute in model 144 is updated to reflect theoff condition in step 216. If no response was received from the networkdevice 44 when it was polled (step 218), a fault isolation inferencehandler is triggered in step 220. The fault isolation inference handleroperates as described below to determine the network component whichcaused network device 44 to fail to respond to the poll. When the causeof the fault is determined, a fault message is sent to the userinterface.

Polling of network device 44 is repeated at intervals specified by anattribute contained in model 144. In addition, other network devices arepolled at intervals which may be different for each network device. Theinformation returned to each model is processed by the inferencehandlers for that model and by inference handlers in other models thatare watching such information. In general, each model type may include adifferent set of inference handlers.

As described above, an attribute change-in one model can trigger aninference handler in one or more other models and thereby produce achain of actions or responses to the attribute change. For example, if afault occurs in a network device, the condition attribute of that deviceis changed in the corresponding model. The condition change may triggera condition change in the model of the room which contains the device.Likewise, the condition change in the room may trigger a conditionchange in the building or site model. The condition attribute in eachmodel may have a different level of significance. For example, failureof a device may have a high significance in the network device model buta relatively low significance in the site model.

The software models and model relations that are representative of anetwork as described herein are highly flexible and adaptable to newnetwork configurations and new management functions. New models andmodel relations are easily added to the virtual network machine toaccommodate the needs of the user. The use of the C++ programminglanguage permits new model types to be derived from existing modeltypes. Thus,-the virtual network machine 12 can be customized for aparticular application. As noted above, the present invention should notbe construed as limited to any particular programming language.

A model type editor is used to modify and control the models in thevirtual network machine 12. The following functions are provided:

(1) Describe () describes some aspect of the specified model type.

(2) Create () creates a new model for the specified model type.

(3) Destroy () removes the specified model from the configuration.

(4) Read () reads the value of the specified attribute from a model.

(5) Write () writes the given values to the attributes of the model.

(6) Action () performs the specified action.

(7) Generate event () creates an event message.

Similarly, the model relations can be edited by the user. The followingfunctions can be performed on model relations.

(1) Describe () describes an aspect of the specified relation.

(2) Read () reads a set of associations.

(3) Add () adds an association.

(4) Remove () removes a set of associations.

(5) Count () returns the number of associations that match the selectioncriteria.

(6) Read rule () reads a set of relation rules.

As indicated above, each inference handler is triggered by theoccurrence of a specified event or events. The user must register theinference handler to receive the trigger. An inference handler can betriggered upon the creation or destruction of a model, the activation orinitializing of a model, the change of an attribute in the same model,the change of an attribute in a watched model, the addition or removalof a relation, the occurrence-of a specified event or a user-definedaction.

The virtual network machine described above including models and modelrelations provides a very general approach to network management. Bycustomizing the virtual network machine, virtually any networkmanagement function can be implemented. Both data (attributes) andintelligence (inference handlers) are encapsulated into a model of anetwork entity. New models can be generated by combining or modifyingexisting models since the models are implemented in the C++programminglanguage. A model can be identified by a variety of different dimensionsor names, depending on the attributes specified. For example, aparticular network device can be identified as a device, a type ofdevice, or by vendor or model number. Models are interrelated with eachother by different types of relations. The relations permitstimulus-response chaining. The model approach provides loosely-coupledintelligent models with interaction between models according tospecified triggers. The system has data location independence. The datafor operation of the virtual network machine may reside in the database,memory or in the physical network which is being modeled.

An important function of a network management system is theidentification and isolation of faults. When the network managementsystem loses contact with a network device, the reason for the loss ofcontact must be determined so that appropriate action, such as a servicecall, can be taken. In a network environment, loss of contact with anetwork device may be due to failure of that network device or tofailure of another network device that is involved in transmission ofthe message. For example, with reference to FIG. 2, assume that contactis lost with network device 53. The loss of contact could be due to thefailure of network device 53, but could also be due to the failure ofnetwork devices 50, 60 or 59. In prior art network management systems,the network administrator was typically provided with a list of possiblecauses of a fault and was required to isolate the fault based on hisexperience and knowledge of the network.

In accordance with a feature of the present invention, the networkmanagement system isolates network faults using a technique known asstatus suppression. When contact between a model and its correspondingnetwork device is lost, the model sets a fault status and initiates thefault isolation technique. According to the fault isolation technique,the model (first model) which lost contact with its correspondingnetwork device (first network device) determines whether adjacent modelshave lost contact with their corresponding network devices. In thiscontext, adjacent network devices are defined as those which aredirectly connected to a specified network device. If adjacent modelscannot contact the corresponding network devices, then the first networkdevice cannot be the cause of the fault, and its fault status in thefirst model is suppressed or overridden. By suppressing the fault statusof the network devices which are determined not to be defective, thedefective network device can be identified.

The fault isolation technique is advantageously implemented in theconjunction with the model-based representation of the network andpolling of network devices as described above. In a preferred embodimentof the fault isolation technique, each model that is capable of pollingits corresponding network device maintains a fault status for thatdevice. If contact with the device is lost, the fault status is set.Each such model also maintains a count of the number of network devicesthat are directly connected to the network device. In addition, eachsuch model maintains a count of the number of adjacent network devicesfor which contact has been lost. This information is determined by eachmodel watching the fault status in models corresponding to adjacentnetwork devices. When a given model loses contact with its correspondingnetwork device, two operations are performed. First, the fault status ofthe model is set. Second, the count of total adjacent devices iscompared with the count of adjacent devices for which the fault statusis set. If the counts are equal, all adjacent models have lost contactwith their corresponding network devices, and the fault status of thefirst model is suppressed.

Since models that are capable of polling network devices perform pollingregularly on an asynchronous basis, the fault status of each such modelis regularly updated. However, when the fault isolation techniquedescribed above is used, the fault status is suppressed in those modelswhich are determined not to be defective. Thus, the fault statuscontained in the models is an accurate representation of defectivenetwork devices.

A flow chart of the fault isolation technique is shown in FIG. 6. When amodel D loses contact with the corresponding network device D (step250), model D sets its fault status in step 252. Model D then obtainsthe fault status of all devices which are adjacent to device D in step252. The fault status of adjacent devices is determined from the faultstatus maintained in models of adjacent devices. In step 256 the numberof adjacent devices N_(A) adjacent to device D is compared with thenumber of adjacent devices having a fault N_(F). If N_(A) is not equalto N_(F), contact can be made with at least one device adjacent todevice D, and the fault status of device D is maintained. If N_(A)=N_(F), contact has been lost with all devices adjacent to device D andthe fault status of device D is suppressed in step 258. As describedabove, this procedure is performed each time a model loses contact withits corresponding network device.

By way of example, assume that model 144 (FIG. 4) is unable to contactits corresponding network device 44 (FIG. 2). The model 144 sets itsfault status and obtains the fault status of adjacent devices 45, 46, 60and 50 from the corresponding models. Assume in this case that the causeof the fault is the bridge device 50. Since the adjacent devices 45, 46,60 and 50 cannot be contacted by the corresponding models, the faultstatus of these devices will be set in the corresponding models. Model144 will therefore determine that the fault status of all adjacentdevices is set and will suppress its own fault status. In this example,the topological configuration of the network and the correspondingmodels are used to isolate the source of a fault and to suppress thefault status of downstream network devices.

The fault isolation technique described above can also be applied in ageographical configuration. For example, assume that contact is lostwith network devices 30, 31 and 34 in room 38 as shown in FIG. 2. Inthis case, it is likely that all devices within the room have failed dueto a power loss or a failure of data bus 36. In this case, the faultstatus of devices 30, 31 and 34 is suppressed in corresponding models130, 131 and 134, and the fault status of room 42 is maintained.

The above examples relate to hardware faults. The fault isolationtechnique of the invention can also be applied to isolation of softwarefaults. As indicated above, the virtual network machine may includemodels of application software running on the network devices. Assume,for example, that contact is lost with an electronic mail applicationrunning on a specified network device. The electronic mail applicationmay depend on other software, such as a file transfer module, for itsoperation. The electronic mail application may have failed to respondbecause of a failure of the file transfer module. The techniquedescribed above can be utilized to isolate the software applicationhaving a fault.

The fault isolation technique described above is particularly useful ina network management system utilizing model-based intelligence asdescribed above. However, the fault isolation technique is not limitedto such use. The fault isolation technique of determining the faultstatus of adjacent devices and suppressing the fault status of the firstdevice when the fault status of all adjacent devices is set, can beapplied in a network management system that does not use models ofnetwork entities. Furthermore, the fault isolation technique is notlimited to network management systems. The technique is more generallyapplicable to any system where it is desired to determine and isolatethe cause of a problem or fault by suppressing symptomatic information.

As indicated above, the user interface 10 provides informationconcerning the network to a user. The primary device for presentingnetwork information to the user is a video display screen. The displayscreen utilizes a high resolution, window-based display system toprovide different views or displays of the network configuration andoperation. The user display is based on the X-Window system whichincludes routines for generating the appropriate display or view basedon input data. The X-Window system is a standard window controllerdeveloped by the X-Consortium at Massachusetts Institute of Technology.The display screen is used in conjunction with a mouse to permit theuser to select different views of the network. It will be understoodthat the user interface can be implemented using other window-basedsystems.

The network management system provides multiple views, includinglocation views, topological views and generic views, of the network.Multifunction icons are used in some views to represent differentnetwork entities. The location and topological views are organized in ahierarchical manner. By clicking on specified elements of a view, theuser can obtain a view of the next lower level in the hierarchy. As usedherein, "clicking" refers to using the mouse to move the cursor to aspecified location on the display screen and then depressing the mousebutton.

In the location views, the highest level may show a map of the worldwith network locations indicated thereon. Intermediate views may show amap of a country or a region, while lower level views may show the floorplan of a building or room that contains network devices. At the lowestlevel, the user may obtain a pictorial view of an individual device.

Examples of location views are shown in FIGS. 7A-7C. A map 300 of thenortheast region, with network locations indicated by icons 302, isshown in FIG. 7A. The icons 302 each include a name label 304 pointingto a circle 306 which indicates a network location. The color of thecircle 306 indicates a status of that location. For example, green mayindicate a normal status, whereas red may indicate a fault or troublestatus. By clicking on one of the network locations, the next lowerlevel location view can be obtained. In this example, a floor plan 310of the headquarters network is shown in FIG. 7B. Locations of networkdevices are indicated by icons 312 which are similar to ions 302described above. By clicking on one of the icons 312 shown in FIG. 7B, alocation view of a single room 318 is displayed as shown in FIG. 7C. Inthis case, the network devices contained within a computer lab arerepresented by multifunction icons 320, 322, which will be described indetail hereinafter.

In the topological views, a similar hierarchy is utilized, and theconnections between network elements are shown. At the highest level,network interconnections at a worldwide or national level are shown. Ateach lower level, more detailed views, such as local area networks andsubnetworks, are shown.

Examples of topological views are shown in FIGS. 8A and 8B. In FIG. 8A,a topological view of a corporate site is shown. An administrationnetwork icon 330 and an engineering network icon 332 are interconnectedto an Internet icon 334 by links 336. Each network is represented by amultifunction icon. By clicking on the engineering network icon 332, aview of the details of the engineering network is obtained, as shown inFIG. 8B. The network devices in the engineering network are representedby multifunction icons 340, 342, 344, and the interconnections 346between network devices are shown.

The location and topological views represent different dimensions of thesame network. The user can traverse between location and topologicalviews to obtain any necessary information regarding the configuration ofthe network. The user display also provides generic views such as analarm log, an event log, a text display, a chart, or any other way ofdisplaying attribute information. The user traverses between availableviews to obtain required network information. There are two basic waysof traversing between views. As indicated above, the user can click onicons in the location and topological views to traverse to the nextlower level in the hierarchy of views. Also, the different views includepull-down menus, as commonly used in window-based displays, which permitselection of any desired view.

Each view that is available to the user has a corresponding view managerin the user interface. Similarly, each icon has a corresponding iconmanager in the user interface. The view manager serves as the commonparent for all parent icon managers associated with a given view. Theview manager saves icon screen placement information and the associatedvirtual network machine model handles that the icons represent. The viewmanager determines other views to which a user may traverse from thecurrent view. The view manager displays appropriate menu items andallows the user to select other views. The view manager may permit theuser to traverse from a location view to a topological view, or viceversa.

The icon manager class is an instantiated C++ class with one or moreicon managers controlling each icon. Each icon manager controls somepart of the on-screen display, such as a bar graph, an arrow or theentire background of an icon. The icon manager represents a model withinthe virtual network machine and contains a representation of the virtualnetwork machine model at the current time. The icon manager cancommunicate with the virtual network machine model that it represents.When attribute data within the virtual network machine model changes,the appropriate icon manager is notified of the change and modifies theicon appearance to reflect the new state, the new statistics orappropriate error conditions. Thus, the icon manager displays data fromthe virtual machine model which it represents.

Icon managers are structured in a hierarchical manner. A parent iconmanager may control a background picture of an icon, and the parenttypically has a group of children icon managers attached it. Each iconmanager has associated with it the model handle of the virtual networkmachine model which it represents.

The icon manager can place a watch on the virtual network machine modelthat it represents. The watch informs the model that an icon manager nowrepresents that model within the user interface. Any changes in thestate of the model are forwarded to the corresponding icon manager. Thewatch includes a parameter that specifies the severity level of thewatch. A change in model attribute data must be equal to or greater thanthe severity level set within the model before the icon manager receivesnotification of a change in attribute data. Another way to place a watchon a virtual network machine model is for the icon manager to set atimer to poll the model periodically. A watch is generic in that thedata received from a watch includes a selected set of attribute data forthe corresponding model. The data in a model may have changedextensively since the icon manager was last notified. When the iconmanager polls a model, it reads attribute data from the model andperforms required actions.

When the user clicks on an icon to proceed to another view, the iconmanager determines the view class and the next view. The icon managerthen issues a new view by passing the view class and the appropriatevirtual network machine model ID to the view executive, thereby causingthe current view to be destroyed.

The user interface 10 and the virtual network machine 12 communicate viaUnix sockets. Messages between these two components are encoded in amachine independent format. A user interface object such as an iconmanager or a view manager may communicate with a model, model type ormodel relation in the virtual network machine in order to retrieveattribute data. It is to be understood that alternative embodiments mayutilize any of a variety of software communication methods and that thepresent invention is in no way limited to any particular operatingsystem or any particular software communication protocol.

The multifunction icons used in the network management system provide ahighly flexible technique for presenting information to the user. Asshown in FIG. 9, a multifunction icon 400 can include an area 402 for adevice name, an area 404 for model type information, bar graphs 406 and408 for indicating performance parameters such as number of packets anderror rate, an area 410 for displaying an iconic or symbolicrepresentation 412 of the device, a background area 414 for representingthe status of the network device by different colors and a FIG. 416 thatis used for traversing to a pictorial representation of the device. Someor all of the areas of the icon can be clicked upon to obtain additionalinformation regarding the network device.

In a preferred embodiment, a view showing general configurationinformation relating to the network device is provided when the userclicks on area 402 or 404 of icon 400. A view showing status informationpertaining to the device is provided when the user clicks on area 410,and a view showing performance information is provided when the userclicks on bar graphs 406 and 408. As indicated above, a pictorialrepresentation of the network device is provided when the user clicks onFIG. 416. It will be understood that the multifunction icon can includedifferent information and areas, depending on the device beingrepresented and the information that is required, and that differentinformation and display views can be provided by clicking on differentareas of the icon.

The multifunction icons shown and described herein are used in an alarmlog view that is shown in FIG. 10. The alarm log view includes an area420 for listing of current alarms, an area 422 for display ofinformation pertaining to a selected alarm and a button panel 423 whichdisplays options available for an alarm. The user may click on aparticular alarm in the listing of current alarms to obtain moreinformation. A multifunction icon 424 representing the network devicehaving a fault is displayed in area 422 with one or more text fields 426and 428 which provide information to the user regarding the cause of thealarm and the status of the device. By clicking on specified areas ofthe icon 424, the user can obtain further information regarding thedevice for which an alarm is registered, as described above inconnection with icon 400. The user can also traverse to the location ortopological view from the alarm log view. By clicking on other alarms inthe alarm list, similar information is obtained regarding other alarmconditions.

The user interface of the network management system is highly flexibleand permits new views of the network to be added to the networkmanagement system. New views require new view managers and icon managersto be instantiated. Since the views are implemented as C++ objects, newviews and icons are easily derived from existing views and icons. Newviews and modifications of existing views are easily provided byadditions or changes to parameters and data which control the views,without changes to the control code.

Certain devices, such as coaxial ("coax") segments, connectors, buses,rings, fiber segments, wide area segments, multiplexers, etc., lack thebuilt-in intelligence to communicate with the network management systemin any manner. Other devices can communicate with the network managementsystem, but only at tremendous cost to the network in terms of networkperformance. Yet, it is desirable for the network management system toobtain information concerning such devices. The present inventionincludes a model and methodology for inferring this information fromother information received by the network management system.

Devices for which direct communication is impossible or impractical arerepresented in the network management system by inferred connectormodels. The inferred connector models construct information withoutpolling the network entity. This is necessary because some networkentities are non-pollable. For other entities, polling istechnologically possible but is extremely costly in terms of its impacton network performance. The inferred connector models differ in somerespects from the models described above. As used herein, the term"connector model" is not limited to models of network connectors, butincludes a model of any network entity in which the information isinferred, as opposed to information obtained by direct communicationwith the entity. As indicated above, such network entities include coaxsegments, connectors, buses, rings, fiber segments, wide area segments,multiplexers, etc. Similarly, the term "connector" is used herein torefer to the above types of network entities. The present invention alsoincludes a methodology wherein polling requests are substantiallyminimized. (How?)

The connector models preferably employ model-based intelligence andinclude inference handlers. By receiving selected information from thenetwork management system, the connector models infer contact status aswell as other information pertaining to the connectors within thenetwork. Information as to the status of a connector (anon-communicating network entity) is inferred from relation orconnection information as to the network entities that are related tothe connector and operational information as to the status of thosenetwork entities that are related to the connector. The operationalinformation is usually obtained by polling. The inferred information canbe accessed or viewed via the user interface, or the trigger otherroutines within the network management system. By providing moreparticular information about the connectors within a network, theconnector models allow the network management system to better diagnosenetwork problems.

In general, the inference handlers of each connector model are triggeredeither when a device is connected to a modeled connector, or by a changein status for any entity that is connected to a modeled connector. Adetailed description of the routines within the connector model isprovided below.

It is useful to define certain terminology that relates to the connectormodels. Contact status is an attribute, i.e., data, within each of themodels. The various states of contact status describe the capability ofthe corresponding network entity to perform on the network. When adevice is "established", the network management system has determinedthat this device is capable of sending and receiving messages. This maybe determined through a simple polling request. When a device is "lost",the network management system has determined that, at least for themoment, the device is unable to respond to polling requests. Asdescribed below, the inability to communicate can be the fault of thedevice itself or it can be the fault of the connector to which thedevice is connected. When a device is in an "initial" state, the networkmanagement system has not yet determined whether the device isestablished or lost. For example, when a device is first added to anetwork, a model is created for that device, and the attributes of themodel are initialized. Among the attributes to be initialized is contactstatus, which receives a value of "initial" and remains in this stateuntil a polling request has been sent to the device to determineotherwise.

From the perspective of the connector models, network devices aredivided into two general categories: non-ported and ported. A personalcomputer (PC), for example, is a non-ported device. The contact statusfor the PC adequately describes the ability of the PC to communicate onthe network. A hub is a ported device, and contact status by itself isinadequate for connector model purposes. Ported devices generally havemany ports, e.g., a hub may have 100 ports. At any given instant, someof the ports may be inoperative, while the remainder continue tofunction. Contact status for the hub does not contain enough informationto indicate the status of the hub ports. Consequently, besides having acontact status to describe the overall status of the ported device, eachport within the ported device has an associated port link status. Thestatus is determined by polling different information within the porteddevice. For example, the contact status is determined by pollinginformation that relates to the hub itself, while port link status isdetermined by polling information particular to that port.

The connector model classifies ports into two types. First, there arerepeater ports. Repeater ports are extremely common entities within anetwork. For instance, a network hub may have 100 repeater ports. Theconnector model, however, requires information from only a relativelyfew of the repeater ports. More specifically, the connector modelrequires information from only those repeater ports that are connectedto a connector with a corresponding inferred connector model. It istherefore advantageous to limit polling requests to those repeater portsthat are connected to modeled connectors. In a preferred embodiment, theconnector models poll only those repeater ports that are connected to amodeled connector. Second, there are Internet Interface ports, which arefar less common than repeater ports in a network system. In thepreferred embodiment all Internet Interface ports are polled, as therelative infrequency of these ports does not warrant the extracomplexity of optimizing software. It is understood, however, that thesame technique applied to repeater port polling optimization can easilybe applied to Internet Interface ports.

The Internet Interface port specific routines utilize names that reflectthe terms used within the art. Specifically, admin₋₋ status andoperational₋₋ status are attributes within the Management InformationBase (MIB) of Internet Interface ports. The connector models utilizethese names. Operational₋₋ status represents the actual status of theport. Admin₋₋ status represents the desired status of the port. Itshould be noted that individual ports can be turned off by themanagement system. When this is done, admin₋₋ status is "down"; admin₋₋status or operational₋₋ status of "up" indicates that the port isoperative.

In the descriptions below, the following naming convention applies.Routine names use capitals and an underscore to connect the wordsforming the name. Data names, or attributes, use lower case. Forinstance, "CHILD₋₋ COUNT" refers to the routine which determines datacalled "child₋₋ count". The actual software comprising the presentinvention can use a different naming methodology and/or different names.

Set forth below are descriptions of the preferred routines whichconstitute the connector model:

1. A CHILD₋₋ COUNT routine counts the number of models that areconnected to the particular connector model. By doing so, the routinetracks the number of actual devices that are connected to thecorresponding connector. Devices can be added and deleted during thelife of the network. Thus, child₋₋ count is dynamic. When a device iseither connected to or disconnected from the connector, the networkmanagement system should be updated via the user interface. When thenetwork management system is updated, this routine is triggered so thatchild₋₋ count is changed accordingly.

2. An INITIAL₋₋ CHILD₋₋ COUNT routine counts the number of models thathave a contact₋₋ status of "initial" and are connected to the particularconnector. This routine has two basic triggering events. First, thisroutine is triggered when a device is connected to or disconnected froma particular connector, and determines whether the connecting ordisconnecting device has a contact₋₋ status of "initial." If it does,initial₋₋ child₋₋ count is changed accordingly. Second, the INITIAL₋₋CHILD₋₋ COUNT routine is triggered when a model that is alreadyconnected to the connector changes its contact₋₋ status. For instance, amodel corresponding to a PC that is connected to a connector starts outwith its contact₋₋ status as "initial." Some time later, after a pollingrequest to the PC, the contact₋₋ status of the PC changes, for example,to "established." When this occurs, INITIAL₋₋ CHILD₋₋ COUNT istriggered, and initial₋₋ child₋₋ count is changed accordingly.

3. A LOST₋₋ CHILD₋₋ COUNT routine has a functionality that is identicalto that for INITIAL₋₋ CHILD₋₋ COUNT with one difference. The differenceis that LOST₋₋ CHILD₋₋ COUNT is concerned with a contact status of"lost" rather than "initial". When a device is connected to ordisconnected from a connector, this routine determines whether thecontact₋₋ status of that device is "lost". Likewise, for any devicesthat are already connected to this connector, the routine determineswhether the contact₋₋ status of the device has changed to or from"lost".

4. A BAD₋₋ LINK₋₋ STATUS₋₋ COUNT routine is concerned with porteddevices and the ports connected to a modeled connector. This routinecounts the number of port models which have a port₋₋ link₋₋ status of"bad" and are connected to the modeled connector. The function of thisroutine is very similar to counting the lost₋₋ child₋₋ count fornon-ported devices. However, connector models are concerned with theindividual status of the ports rather than the overall contact₋₋ statusof the ported device.

5. A REPEATER₋₋ PORT₋₋ LINK₋₋ STATUS routine determines the port₋₋link₋₋ status for all repeater ports that are connected to an inferredconnector model. This routine obviates the need for polling eachrepeater port in the ported device. It does so by watching certainpolling control variables within the network management system. Fromthis information, this routine determines when the ported device is tobe polled. At the polling interval, this routine reads the total trafficobject for only the connected repeater ports. The total traffic objectis a data value that can be read from each repeater port and thatindicates the volume of traffic through that port. If there is a changein the traffic count from the last polled value, this routine infersthat the port₋₋ link₋₋ status is "good." If there is no change in thetraffic count, this routine sets port₋₋ link₋₋ status to "bad." Itshould be noted that when contact₋₋ status is "lost" for the porteddevice itself, port₋₋ link₋₋ status for the individual ports is set to"unknown." This action is taken so that double counting does not occurwhen computing the number of operative devices or ports connected to amodeled connector. This is more fully described below in the discussionof COMPOSITE₋₋ BAD₋₋ COUNT.

6. An INTERFACE₋₋ INTERNAL₋₋ LINK₋₋ STATUS routine determines the port₋₋link₋₋ status for Internet Interface ports that are connected to amodeled connector. This routine polls operational₋₋ status for thoseports. When the operational₋₋ status is "down" and the admin₋₋ status is"up" after polling, this routine sets port₋₋ link₋₋ status to "bad";otherwise port₋₋ link₋₋ status is set to "good". It should be noted thata port can be turned off by the management system. When this is done,admin₋₋ status is set to "down." It follows that for connector modelpurposes, when the desired status, i.e., admin₋₋ status, is down for aparticular port, an operational₋₋ status of down for that port shouldnot be construed as the port being inoperative. For the reasonsdiscussed above, when contact₋₋ status is "lost" for the ported device,port₋₋ link₋₋ status is set to "unknown."

7. A COMPOSITE₋₋ BAD₋₋ COUNT routine computes the sum of bad₋₋ link₋₋status₋₋ count and lost₋₋ child₋₋ count for the particular connectormodel. When either bad₋₋ link₋₋ status₋₋ count or lost₋₋ child₋₋ countchanges, COMPOSITE₋₋ BAD₋₋ COUNT is triggered and composite₋₋ bad₋₋count is changed accordingly. Within the virtual network, both a porteddevice and an individual port within that device can be regarded asconnected to the same modeled connector. Yet, in reality there is onlyone device connected to the connector. When the contact₋₋ status of theported device changes to lost, LOST₋₋ CHILD₋₋ COUNT for that connectormodel is triggered. At the same time, the REPEATER₋₋ PORT₋₋ LINK₋₋STATUS and INTERFACE₋₋ INTERNAL₋₋ LINK₋₋ STATUS routines discussed abovechange port₋₋ link₋₋ status to "unknown." This is done to prevent theport₋₋ link₋₋ status from being- changed to bad and thereby preventsdouble counting. For example, when a hub has one of its ports connectedto a modeled connector, both the hub and the port have a modelrelationship indicating that they are connected to the connector. Whenthe hub loses contact status, LOST₋₋ CHILD₋₋ COUNT for the connector istriggered and consequently COMPOSITE₋₋ BAD₋₋ COUNT is also triggered. Inaddition, it is highly likely that when the hub loses contact status allthe ports will become "bad." If the previously discussed routines didnot change the status to "unknown," a "bad" port status likewisetriggers COMPOSITE₋₋ BAD₋₋ COUNT. The result is that, although inreality only one connection to the modeled connector has becomeinoperative, COMPOSITE₋₋ BAD₋₋ COUNT would indicate that two connectionshave become "bad".

8. A CONTACT₋₋ STATUS routine infers the contact₋₋ status of theparticular connector model. The result of this routine is an inferenceof whether the actual connector being modeled is operational, i.e.,established. The contact₋₋ status for the particular connector model isdetermined by the following formula:

If ((composite₋₋ bad₋₋ count GTR=(child₋₋ count-initial₋₋ child₋₋count)) and (composite₋₋ bad₋₋ count GTR 0)) contact₋₋ status=lost

else if (initial₋₋ child₋₋ count EQL child₋₋ count) contact₋₋status=initial

else contact₋₋ status=established,

where "GTR=" means greater than or equal to, and "EQL" means equal to.

The formula makes the following inferences. First, if all entitiesconnected to a connector are either "lost," known "bad," or "initial,"the connector is inferred to be "lost." This inference is sound because,if the connector is "lost," this can account for all of the deviceshaving their contact status as "lost" or their port₋₋ link₋₋ status as"bad." Second, if all the devices on the connector are still in aninitial state, then the connector is best described as being in aninitial state, i.e., it is not yet known whether the connector isproperly connected. It should be noted that models do not remain in an"initial" state for very long. Contact₋₋ status changes from "initial"after the next polling interval. Polling intervals ordinarily occur onthe order of every minute, but as previously stated the polling intervalis programmable. Finally, if any device connected to the connector is"established", then the connector must be established, as there is noother way in which the device could have that contact₋₋ status.

The following examples illustrate the connector model in-operation.

Referring to FIG. 11, five pollable devices 510, 520, 530, 540, 550(including four PC's and one bridge) are connected to a coax segment500. Each of the pollable devices 510, 520, 530, 540, 550 has acorresponding model within the previously described virtual network. Amodel of the coax segment 500 is an inferred connector model. The modelof each device has an attribute to reflect the current, or at least thelast polled, contact status. Assume models of all five devices have acontact₋₋ status of "initial." The routines described above infer thatthe coax segment 500 has a contact status of "initial." Some time later,the devices will have performed some actual work. The network managementsystem, likewise, will have polled these devices to determine theirstatus.

Referring to FIG. 12, the devices 520, 530, 540 and 550 connected to thecoax segment 500 now have a contact₋₋ status of "lost"; the device 510has a contact status of "established". Since it is impossible for thedevice 510 to have a contact₋₋ status of "established" without having afunctioning coax segment to determine this, the coax segment 500likewise must have an inferred contact status of "established." Thecontact₋₋ status of the device 510 is determined by polling requeststhat travel via the coax segment 500.

Now assume that at the next polling interval no response is receivedfrom PC 510. The current state of the network is illustrated in FIG. 13.When no response is received from PC 510, the following changes occur inthe virtual network. Contact₋₋ status within the model for PC 510changes to "lost". This change triggers LOST₋₋ CHILD₋₋ COUNT in themodel for the coax segment 500. LOST₋₋ CHILD₋₋ COUNT, accordingly,changes the data value lost₋₋ child₋₋ count. The change in lost₋₋child₋₋ count triggers COMPOSITE₋₋ BAD₋₋ COUNT within the same model.COMPOSITE₋₋ BAD₋₋ COUNT changes the data value composite₋₋ bad₋₋ countaccordingly. The change in composite₋₋ bad₋₋ count triggers CONTACT₋₋STATUS within the same model. Finally, the contact₋₋ status for the coaxsegment 500 changes to "lost," as all the devices connected to that coaxsegment are now lost. This change in contact₋₋ status can then be sentto the network management system where it is likely to be immediatelyrelayed to the video display as an alarm. It should be noted that thecoax segment 500 is not necessarily defective. However, an intelligentinference, made by the connector models, reasons that all five devices510, 520, 530, 540, 550 have a contact₋₋ status of "lost" because thecoax segment 500 connecting them together is "lost." It is more probablethat the coax segment 500 is a single point of failure than that fivedevices have independently and concurrently become inoperable.

It should be noted that the inferred connector models described hereinare not limited to use in the network management system described above.The connector model can operate with any network management system thatis capable of supplying the required information to it. The networkmanagement system must be capable of providing information when newdevices are added to or deleted from the connector and when the statusof any connected device changes.

While there have been shown and described what are at present consideredthe preferred embodiments of the present invention, it will be obviousto those skilled in the art that various changes and modifications maybe made therein without departing from the scope of the invention asdefined by the appended claims.

What is claimed is:
 1. In a network comprising interrelated networkentities including communicating network entities and at least onenon-communicating network entity, a system for monitoring a status ofthe non-communicating network entity, comprising:means for providing anelectronic network management system; means for said network managementsystem communicating with said communicating network entities to obtainoperational information as to the communicating network entities; andmeans for said network management system using the operationalinformation as to the communicating network entities and information asto relations between said non-communicating network entity and saidcommunicating network entities to infer the status of thenon-communicating network entity.
 2. A system as defined in claim 1wherein said means for communicating with said communicating networkentities includes means for said network management system polling saidnetwork entities at predetermined intervals.
 3. A system as defined inclaim 1 wherein said means for providing an electronic networkmanagement system includes means for maintaining in the networkmanagement system a representation of the network including models andrelations between models, said models including models of saidcommunicating network entities and inferred models of saidnon-communicating network entity, each model including one or moreinference handlers, said inference handlers in said inferred modelsupdating status information in response to information obtained from themodels of said communicating network entities.
 4. A system as defined inclaim 1 further including means for providing the status of thenon-communicating network entity to a user.
 5. A system as defined inclaim 1 wherein means for inferring the status of the non-communicatingnetwork entity includes inferring a lost contact status when all networkentities connected to the non-communicating network entity have a lostcontact status.
 6. A system as defined in claim 1 wherein said means forinferring the status of the non-communicating network entity includesmeans for inferring an established contact status when at least onenetwork entity connected to the non-communicating network entity has anestablished contact status.
 7. A system as defined in claim 1 whereinsaid means for inferring the status of the non-communicating entityincludes means for inferring an initial contact status when all networkentities connected to the non-communicating network entity have aninitial contact status.
 8. A system as defined in claim 1 furtherincluding means for updating the information as to relations betweensaid non-communicating network entity and said communicating networkentities when a network entity is added to or removed from said network.9. A system as defined in claim 1 wherein said means for obtainingoperational information as to the communicating network entitiesincludes means for periodically updating a contact status of saidcommunicating network entities.
 10. A system as defined in claim 1wherein said means for obtaining operational information as to thecommunicating network entities includes means for updating a contactstatus of ports in a ported network entity.